WARNING: This document has been automatically Deferred after 12 months of inactivity in its previous Experimental state. Implementation of the protocol described herein is not recommended for production systems. However, exploratory implementations are encouraged to resume the standards process.

Unlike similar XEPs, e.g., OMEMO Encryption (XEP-0384) [2], this XEP does not
provide Forward Secrecy (FS), but as an advantage in return, allows
users to read their archived conversations (respectively their
encrypted data) later on. Of course, only as long as they still
possess the according secret key. FS and being able to decrypt
archived messages are mutually exclusive, i.e., one can not have
both. The authors therefore consider this XEP complementary to
similar ones which also provide end-to-end encryption but with a
different feature set.

If an entity supports exchanging OpenPGP encrypted and signed
instant messages over XMPP, i.e., what is specified herein, it MUST
advertise that fact by announcing a Service Discovery (XEP-0030) [3] feature of
'urn:xmpp:openpgp:im:0'. It thus includes this feature in response
to a service discovery request.

In order to establish an OpenPGP secured IM communication, IM
clients first need to determine the public key of their
interlocutor(s). OpenPGP historically provides public keyservers
which can be used for key retrieval. Additional there are methods
to store OpenPGP key information in the Domain Name
System (DNS). This specification does not restrict the mechanism
of key discovery and retrieval, but compliant clients MUST support
the public key announcement as described in XEP-0373§ 4.

After the required public keys have been discovered, XMPP
clients engage in an OpenPGP secured IM
conversation by exchanging <openpgp/> extension elements. They MUST
use the <signcrypt/> OpenPGP content element specified in
XEP-0373§ 3.1.

The child elements of the OpenPGP content element's <payload/>
can be seen as stanza extension elements which are encrypted and
signed. After the <openpgp/> element and the including <signcrypt/>,
element was verified, they SHOULD be processed similar as if they
had been direct extension elements of the stanza. For example,
direct child elements found in <payload/> in the context of IM
could be:

But just as with stanza extension elements, child elements of
<payload/> can be any extension element. The example above uses
the <body/> element as defined in RFC 6121. Note that it
uses 'jabber:client' as namespace, but since the same
<body/> element is also defined in the 'jabber:server'
namespace, recipients MUST accept both.

Clients MAY want to use the mechanism in XEP-0373 § 5 to
synchronize their secret key(s) over multiple devices. Thus, they
should query the user's PEP service for an eventually stored
encrypted secret key.

Only <signcrypt/> MUST be used for the IM
use case. Encrypted but unsigned messages (<crypt/>) do not provide
an advantage over unencrypted ones since the sender can not be
verified. As result of this rule, the user interface of
IM clients implementing the protocol defined herein MUST NOT
provide an option for the user to select between sign+crypt, sign
or crypt. This also increases the usability.

In the IM use case every <message/> equipped with <openpgp/>
SHOULD include an unencrypted <body/> explaining that the actual
message is encrypted. Furthermore the message SHOULD contain a
'store' hint as defined in Message Processing Hints (XEP-0334) [7] §
4.4 and a "this message contains an encrypted body text" hint
in form of an <encryption/> extension element as specified by
Explicit Message Encryption (XEP-0380) [8].

Vincent Breitmoser

Appendix C: Legal Notices

Copyright

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

9. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

10. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.